# Standard library header <flat_map> (C++23)

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This header is part of the containers library.

### Includes
<compare> (C++20) | Three-way comparison operator support
<initializer_list> (C++11) | std::initializer_list class template
### Classes
flat_map (C++23) | adapts two containers to provide a collection of key-value pairs, sorted by unique keys (class template)
flat_multimap (C++23) | adapts two containers to provide a collection of key-value pairs, sorted by keys (class template)
std::uses_allocator<std::flat_map> (C++23) | specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::flat_multimap> (C++23) | specializes the std::uses_allocator type trait (class template specialization)
### Functions
erase_if(std::flat_map) (C++23) | erases all elements satisfying specific criteria (function template)
erase_if(std::flat_multimap) (C++23) | erases all elements satisfying specific criteria (function template)
### Tags
sorted_uniquesorted_unique_t (C++23) | indicates that elements of a range are sorted and unique (tag)
sorted_equivalentsorted_equivalent_t (C++23) | indicates that elements of a range are sorted (uniqueness is not required) (tag)

### Synopsis

    #include <compare>
    #include <initializer_list>

    namespace std {
      // class template flat_map
      template<class Key,
               class T,
               class Compare         = less<Key>,
               class KeyContainer    = vector<Key>,
               class MappedContainer = vector<T>>
      class flat_map;

      struct sorted_unique_t
      {
        explicit sorted_unique_t() = default;
      };
      inline constexpr sorted_unique_t sorted_unique{};

      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Allocator>
      struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                            Allocator>;

      // erasure for flat_map
      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Predicate>
      typename flat_map<Key, T, Compare, KeyContainer, MappedContainer>::size_type erase_if(
        flat_map<Key, T, Compare, KeyContainer, MappedContainer>& c,
        Predicate pred);

      // class template flat_multimap
      template<class Key,
               class T,
               class Compare         = less<Key>,
               class KeyContainer    = vector<Key>,
               class MappedContainer = vector<T>>
      class flat_multimap;

      struct sorted_equivalent_t
      {
        explicit sorted_equivalent_t() = default;
      };
      inline constexpr sorted_equivalent_t sorted_equivalent{};

      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Allocator>
      struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                            Allocator>;

      // erasure for flat_multimap
      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Predicate>
      typename flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>::size_type
      erase_if(flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>& c,
               Predicate pred);
    }

#### Class template std::flat_map

    namespace std {
      template<class Key,
               class T,
               class Compare         = less<Key>,
               class KeyContainer    = vector<Key>,
               class MappedContainer = vector<T>>
      class flat_map
      {
      public:
        // types
        using key_type               = Key;
        using mapped_type            = T;
        using value_type             = pair<key_type, mapped_type>;
        using key_compare            = Compare;
        using reference              = pair<const key_type&, mapped_type&>;
        using const_reference        = pair<const key_type&, const mapped_type&>;
        using size_type              = size_t;
        using difference_type        = ptrdiff_t;
        using iterator               = /* implementation-defined */;
        using const_iterator         = /* implementation-defined */;
        using reverse_iterator       = std::reverse_iterator<iterator>;
        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
        using key_container_type     = KeyContainer;
        using mapped_container_type  = MappedContainer;

        class value_compare
        {
        private:
          key_compare comp; // exposition only
          value_compare(key_compare c)
            : comp(c)
          {
          } // exposition only

        public:
          bool operator()(const_reference x, const_reference y) const
          {
            return comp(x.first, y.first);
          }
        };

        struct containers
        {
          key_container_type keys;
          mapped_container_type values;
        };

        // constructors
        flat_map()
          : flat_map(key_compare())
        {
        }

        explicit flat_map(const key_compare& comp)
          : c()
          , compare(comp)
        {
        }

        flat_map(key_container_type key_cont,
                 mapped_container_type mapped_cont,
                 const key_compare& comp = key_compare());

        flat_map(sorted_unique_t,
                 key_container_type key_cont,
                 mapped_container_type mapped_cont,
                 const key_compare& comp = key_compare());

        template<class InputIter>
        flat_map(InputIter first, InputIter last, const key_compare& comp = key_compare())
          : c()
          , compare(comp)
        {
          insert(first, last);
        }

        template<class InputIter>
        flat_map(sorted_unique_t s,
                 InputIter first,
                 InputIter last,
                 const key_compare& comp = key_compare())
          : c()
          , compare(comp)
        {
          insert(s, first, last);
        }

        template<container-compatible-range<value_type> R>
        flat_map(from_range_t fr, R&& rg)
          : flat_map(fr, std::forward<R>(rg), key_compare())
        {
        }
        template<container-compatible-range<value_type> R>
        flat_map(from_range_t, R&& rg, const key_compare& comp)
          : flat_map(comp)
        {
          insert_range(std::forward<R>(rg));
        }

        flat_map(initializer_list<value_type> il, const key_compare& comp = key_compare())
          : flat_map(il.begin(), il.end(), comp)
        {
        }

        flat_map(sorted_unique_t s,
                 initializer_list<value_type> il,
                 const key_compare& comp = key_compare())
          : flat_map(s, il.begin(), il.end(), comp)
        {
        }

        // constructors with allocators

        template<class Alloc>
        explicit flat_map(const Alloc& a);
        template<class Alloc>
        flat_map(const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_map(const key_container_type& key_cont,
                 const mapped_container_type& mapped_cont,
                 const Alloc& a);
        template<class Alloc>
        flat_map(const key_container_type& key_cont,
                 const mapped_container_type& mapped_cont,
                 const key_compare& comp,
                 const Alloc& a);
        template<class Alloc>
        flat_map(sorted_unique_t,
                 const key_container_type& key_cont,
                 const mapped_container_type& mapped_cont,
                 const Alloc& a);
        template<class Alloc>
        flat_map(sorted_unique_t,
                 const key_container_type& key_cont,
                 const mapped_container_type& mapped_cont,
                 const key_compare& comp,
                 const Alloc& a);
        template<class Alloc>
        flat_map(const flat_map&, const Alloc& a);
        template<class Alloc>
        flat_map(flat_map&&, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_map(InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_map(InputIter first, InputIter last, const key_compare& comp, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_map(sorted_unique_t, InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_map(sorted_unique_t,
                 InputIter first,
                 InputIter last,
                 const key_compare& comp,
                 const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_map(from_range_t, R&& rg, const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_map(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_map(initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_map(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_map(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_map(sorted_unique_t,
                 initializer_list<value_type> il,
                 const key_compare& comp,
                 const Alloc& a);

        flat_map& operator=(initializer_list<value_type>);

        // iterators
        iterator begin() noexcept;
        const_iterator begin() const noexcept;
        iterator end() noexcept;
        const_iterator end() const noexcept;

        reverse_iterator rbegin() noexcept;
        const_reverse_iterator rbegin() const noexcept;
        reverse_iterator rend() noexcept;
        const_reverse_iterator rend() const noexcept;

        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;
        const_reverse_iterator crbegin() const noexcept;
        const_reverse_iterator crend() const noexcept;

        // capacity
        bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // element access
        mapped_type& operator[](const key_type& x);
        mapped_type& operator[](key_type&& x);
        template<class K>
        mapped_type& operator[](K&& x);
        mapped_type& at(const key_type& x);
        const mapped_type& at(const key_type& x) const;
        template<class K>
        mapped_type& at(const K& x);
        template<class K>
        const mapped_type& at(const K& x) const;

        // modifiers
        template<class... Args>
        pair<iterator, bool> emplace(Args&&... args);
        template<class... Args>
        iterator emplace_hint(const_iterator position, Args&&... args);

        pair<iterator, bool> insert(const value_type& x) { return emplace(x); }
        pair<iterator, bool> insert(value_type&& x) { return emplace(std::move(x)); }
        iterator insert(const_iterator position, const value_type& x)
        {
          return emplace_hint(position, x);
        }
        iterator insert(const_iterator position, value_type&& x)
        {
          return emplace_hint(position, std::move(x));
        }

        template<class P>
        pair<iterator, bool> insert(P&& x);
        template<class P>
        iterator insert(const_iterator position, P&&);
        template<class InputIter>
        void insert(InputIter first, InputIter last);
        template<class InputIter>
        void insert(sorted_unique_t, InputIter first, InputIter last);
        template<container-compatible-range<value_type> R>
        void insert_range(R&& rg);

        void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
        void insert(sorted_unique_t s, initializer_list<value_type> il)
        {
          insert(s, il.begin(), il.end());
        }

        containers extract() &&;
        void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont);

        template<class... Args>
        pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
        template<class... Args>
        pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
        template<class K, class... Args>
        pair<iterator, bool> try_emplace(K&& k, Args&&... args);
        template<class... Args>
        iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
        template<class... Args>
        iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
        template<class K, class... Args>
        iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
        template<class M>
        pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
        template<class M>
        pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
        template<class K, class M>
        pair<iterator, bool> insert_or_assign(K&& k, M&& obj);
        template<class M>
        iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
        template<class M>
        iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
        template<class K, class M>
        iterator insert_or_assign(const_iterator hint, K&& k, M&& obj);

        iterator erase(iterator position);
        iterator erase(const_iterator position);
        size_type erase(const key_type& x);
        template<class K>
        size_type erase(K&& x);
        iterator erase(const_iterator first, const_iterator last);

        void swap(flat_map& y) noexcept;
        void clear() noexcept;

        // observers
        key_compare key_comp() const;
        value_compare value_comp() const;

        const key_container_type& keys() const noexcept { return c.keys; }
        const mapped_container_type& values() const noexcept { return c.values; }

        // map operations
        iterator find(const key_type& x);
        const_iterator find(const key_type& x) const;
        template<class K>
        iterator find(const K& x);
        template<class K>
        const_iterator find(const K& x) const;

        size_type count(const key_type& x) const;
        template<class K>
        size_type count(const K& x) const;

        bool contains(const key_type& x) const;
        template<class K>
        bool contains(const K& x) const;

        iterator lower_bound(const key_type& x);
        const_iterator lower_bound(const key_type& x) const;
        template<class K>
        iterator lower_bound(const K& x);
        template<class K>
        const_iterator lower_bound(const K& x) const;

        iterator upper_bound(const key_type& x);
        const_iterator upper_bound(const key_type& x) const;
        template<class K>
        iterator upper_bound(const K& x);
        template<class K>
        const_iterator upper_bound(const K& x) const;

        pair<iterator, iterator> equal_range(const key_type& x);
        pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
        template<class K>
        pair<iterator, iterator> equal_range(const K& x);
        template<class K>
        pair<const_iterator, const_iterator> equal_range(const K& x) const;

        friend bool operator==(const flat_map& x, const flat_map& y);

        friend /*synth-three-way-result*/<value_type> operator<=>(const flat_map& x,
                                                                  const flat_map& y);

        friend void swap(flat_map& x, flat_map& y) noexcept { x.swap(y); }

      private:
        containers c;        // exposition only
        key_compare compare; // exposition only

        struct key_equiv
        { // exposition only
          key_equiv(key_compare c)
            : comp(c)
          {
          }
          bool operator()(const_reference x, const_reference y) const
          {
            return !comp(x.first, y.first) && !comp(y.first, x.first);
          }
          key_compare comp;
        };
      };

      template<class KeyContainer,
               class MappedContainer,
               class Compare = less<typename KeyContainer::value_type>>
      flat_map(KeyContainer, MappedContainer, Compare = Compare())
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    Compare,
                    KeyContainer,
                    MappedContainer>;

      template<class KeyContainer, class MappedContainer, class Allocator>
      flat_map(KeyContainer, MappedContainer, Allocator)
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    less<typename KeyContainer::value_type>,
                    KeyContainer,
                    MappedContainer>;
      template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
      flat_map(KeyContainer, MappedContainer, Compare, Allocator)
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    Compare,
                    KeyContainer,
                    MappedContainer>;

      template<class KeyContainer,
               class MappedContainer,
               class Compare = less<typename KeyContainer::value_type>>
      flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare = Compare())
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    Compare,
                    KeyContainer,
                    MappedContainer>;

      template<class KeyContainer, class MappedContainer, class Allocator>
      flat_map(sorted_unique_t, KeyContainer, MappedContainer, Allocator)
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    less<typename KeyContainer::value_type>,
                    KeyContainer,
                    MappedContainer>;
      template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
      flat_map(sorted_unique_t, KeyContainer, MappedContainer, Compare, Allocator)
        -> flat_map<typename KeyContainer::value_type,
                    typename MappedContainer::value_type,
                    Compare,
                    KeyContainer,
                    MappedContainer>;

      template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
      flat_map(InputIter, InputIter, Compare = Compare())
        -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

      template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
      flat_map(sorted_unique_t, InputIter, InputIter, Compare = Compare())
        -> flat_map</*iter-key-type*/<InputIter>, /*iter-mapped-type*/<InputIter>, Compare>;

      template<ranges::input_range R,
               class Compare   = less</*range-key-type*/<R>>,
               class Allocator = allocator<byte>>
      flat_map(from_range_t, R&&, Compare = Compare(), Allocator = Allocator()) -> flat_map<
        /*range-key-type*/<R>,
        /*range-mapped-type*/<R>,
        Compare,
        vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
        vector</*range-mapped-type*/<R>,
               /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

      template<ranges::input_range R, class Allocator>
      flat_map(from_range_t, R&&, Allocator) -> flat_map<
        /*range-key-type*/<R>,
        /*range-mapped-type*/<R>,
        less</*range-key-type*/<R>>,
        vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
        vector</*range-mapped-type*/<R>,
               /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

      template<class Key, class T, class Compare = less<Key>>
      flat_map(initializer_list<pair<Key, T>>, Compare = Compare())
        -> flat_map<Key, T, Compare>;

      template<class Key, class T, class Compare = less<Key>>
      flat_map(sorted_unique_t, initializer_list<pair<Key, T>>, Compare = Compare())
        -> flat_map<Key, T, Compare>;

      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Allocator>
      struct uses_allocator<flat_map<Key, T, Compare, KeyContainer, MappedContainer>,
                            Allocator>
        : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                        uses_allocator_v<MappedContainer, Allocator>>
      {};
    }

#### Class template std::flat_multimap

    namespace std {
      template<class Key,
               class T,
               class Compare         = less<Key>,
               class KeyContainer    = vector<Key>,
               class MappedContainer = vector<T>>
      class flat_multimap
      {
      public:
        // types
        using key_type               = Key;
        using mapped_type            = T;
        using value_type             = pair<key_type, mapped_type>;
        using key_compare            = Compare;
        using reference              = pair<const key_type&, mapped_type&>;
        using const_reference        = pair<const key_type&, const mapped_type&>;
        using size_type              = size_t;
        using difference_type        = ptrdiff_t;
        using iterator               = /* implementation-defined */;
        using const_iterator         = /* implementation-defined */;
        using reverse_iterator       = std::reverse_iterator<iterator>;
        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
        using key_container_type     = KeyContainer;
        using mapped_container_type  = MappedContainer;

        class value_compare
        {
        private:
          key_compare comp; // exposition only
          value_compare(key_compare c)
            : comp(c)
          {
          } // exposition only

        public:
          bool operator()(const_reference x, const_reference y) const
          {
            return comp(x.first, y.first);
          }
        };

        struct containers
        {
          key_container_type keys;
          mapped_container_type values;
        };

        // constructors
        flat_multimap()
          : flat_multimap(key_compare())
        {
        }

        explicit flat_multimap(const key_compare& comp)
          : c()
          , compare(comp)
        {
        }

        flat_multimap(key_container_type key_cont,
                      mapped_container_type mapped_cont,
                      const key_compare& comp = key_compare());

        flat_multimap(sorted_equivalent_t,
                      key_container_type key_cont,
                      mapped_container_type mapped_cont,
                      const key_compare& comp = key_compare());

        template<class InputIter>
        flat_multimap(InputIter first,
                      InputIter last,
                      const key_compare& comp = key_compare())
          : c()
          , compare(comp)
        {
          insert(first, last);
        }

        template<class InputIter>
        flat_multimap(sorted_equivalent_t s,
                      InputIter first,
                      InputIter last,
                      const key_compare& comp = key_compare())
          : c()
          , compare(comp)
        {
          insert(s, first, last);
        }

        template<container-compatible-range<value_type> R>
        flat_multimap(from_range_t fr, R&& rg)
          : flat_multimap(fr, std::forward<R>(rg), key_compare())
        {
        }
        template<container-compatible-range<value_type> R>
        flat_multimap(from_range_t, R&& rg, const key_compare& comp)
          : flat_multimap(comp)
        {
          insert_range(std::forward<R>(rg));
        }

        flat_multimap(initializer_list<value_type> il,
                      const key_compare& comp = key_compare())
          : flat_multimap(il.begin(), il.end(), comp)
        {
        }

        flat_multimap(sorted_equivalent_t s,
                      initializer_list<value_type> il,
                      const key_compare& comp = key_compare())
          : flat_multimap(s, il.begin(), il.end(), comp)
        {
        }

        // constructors with allocators

        template<class Alloc>
        explicit flat_multimap(const Alloc& a);
        template<class Alloc>
        flat_multimap(const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_multimap(const key_container_type& key_cont,
                      const mapped_container_type& mapped_cont,
                      const Alloc& a);
        template<class Alloc>
        flat_multimap(const key_container_type& key_cont,
                      const mapped_container_type& mapped_cont,
                      const key_compare& comp,
                      const Alloc& a);
        template<class Alloc>
        flat_multimap(sorted_equivalent_t,
                      const key_container_type& key_cont,
                      const mapped_container_type& mapped_cont,
                      const Alloc& a);
        template<class Alloc>
        flat_multimap(sorted_equivalent_t,
                      const key_container_type& key_cont,
                      const mapped_container_type& mapped_cont,
                      const key_compare& comp,
                      const Alloc& a);
        template<class Alloc>
        flat_multimap(const flat_multimap&, const Alloc& a);
        template<class Alloc>
        flat_multimap(flat_multimap&&, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multimap(InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multimap(InputIter first,
                      InputIter last,
                      const key_compare& comp,
                      const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multimap(sorted_equivalent_t, InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multimap(sorted_equivalent_t,
                      InputIter first,
                      InputIter last,
                      const key_compare& comp,
                      const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_multimap(from_range_t, R&& rg, const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_multimap(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_multimap(initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_multimap(initializer_list<value_type> il,
                      const key_compare& comp,
                      const Alloc& a);
        template<class Alloc>
        flat_multimap(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_multimap(sorted_equivalent_t,
                      initializer_list<value_type> il,
                      const key_compare& comp,
                      const Alloc& a);

        flat_multimap& operator=(initializer_list<value_type>);

        // iterators
        iterator begin() noexcept;
        const_iterator begin() const noexcept;
        iterator end() noexcept;
        const_iterator end() const noexcept;

        reverse_iterator rbegin() noexcept;
        const_reverse_iterator rbegin() const noexcept;
        reverse_iterator rend() noexcept;
        const_reverse_iterator rend() const noexcept;

        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;
        const_reverse_iterator crbegin() const noexcept;
        const_reverse_iterator crend() const noexcept;

        // capacity
        bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // modifiers
        template<class... Args>
        iterator emplace(Args&&... args);
        template<class... Args>
        iterator emplace_hint(const_iterator position, Args&&... args);

        iterator insert(const value_type& x) { return emplace(x); }
        iterator insert(value_type&& x) { return emplace(std::move(x)); }
        iterator insert(const_iterator position, const value_type& x)
        {
          return emplace_hint(position, x);
        }
        iterator insert(const_iterator position, value_type&& x)
        {
          return emplace_hint(position, std::move(x));
        }

        template<class P>
        iterator insert(P&& x);
        template<class P>
        iterator insert(const_iterator position, P&&);
        template<class InputIter>
        void insert(InputIter first, InputIter last);
        template<class InputIter>
        void insert(sorted_equivalent_t, InputIter first, InputIter last);
        template<container-compatible-range<value_type> R>
        void insert_range(R&& rg);

        void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
        void insert(sorted_equivalent_t s, initializer_list<value_type> il)
        {
          insert(s, il.begin(), il.end());
        }

        containers extract() &&;
        void replace(key_container_type&& key_cont, mapped_container_type&& mapped_cont);

        iterator erase(iterator position);
        iterator erase(const_iterator position);
        size_type erase(const key_type& x);
        template<class K>
        size_type erase(K&& x);
        iterator erase(const_iterator first, const_iterator last);

        void swap(flat_multimap&) noexcept;
        void clear() noexcept;

        // observers
        key_compare key_comp() const;
        value_compare value_comp() const;

        const key_container_type& keys() const noexcept { return c.keys; }
        const mapped_container_type& values() const noexcept { return c.values; }

        // map operations
        iterator find(const key_type& x);
        const_iterator find(const key_type& x) const;
        template<class K>
        iterator find(const K& x);
        template<class K>
        const_iterator find(const K& x) const;

        size_type count(const key_type& x) const;
        template<class K>
        size_type count(const K& x) const;

        bool contains(const key_type& x) const;
        template<class K>
        bool contains(const K& x) const;

        iterator lower_bound(const key_type& x);
        const_iterator lower_bound(const key_type& x) const;
        template<class K>
        iterator lower_bound(const K& x);
        template<class K>
        const_iterator lower_bound(const K& x) const;

        iterator upper_bound(const key_type& x);
        const_iterator upper_bound(const key_type& x) const;
        template<class K>
        iterator upper_bound(const K& x);
        template<class K>
        const_iterator upper_bound(const K& x) const;

        pair<iterator, iterator> equal_range(const key_type& x);
        pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
        template<class K>
        pair<iterator, iterator> equal_range(const K& x);
        template<class K>
        pair<const_iterator, const_iterator> equal_range(const K& x) const;

        friend bool operator==(const flat_multimap& x, const flat_multimap& y);

        friend /*synth-three-way-result*/<value_type> operator<=>(const flat_multimap& x,
                                                                  const flat_multimap& y);

        friend void swap(flat_multimap& x, flat_multimap& y) noexcept { x.swap(y); }

      private:
        containers c;        // exposition only
        key_compare compare; // exposition only
      };

      template<class KeyContainer,
               class MappedContainer,
               class Compare = less<typename KeyContainer::value_type>>
      flat_multimap(KeyContainer, MappedContainer, Compare = Compare())
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         Compare,
                         KeyContainer,
                         MappedContainer>;

      template<class KeyContainer, class MappedContainer, class Allocator>
      flat_multimap(KeyContainer, MappedContainer, Allocator)
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         less<typename KeyContainer::value_type>,
                         KeyContainer,
                         MappedContainer>;
      template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
      flat_multimap(KeyContainer, MappedContainer, Compare, Allocator)
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         Compare,
                         KeyContainer,
                         MappedContainer>;

      template<class KeyContainer,
               class MappedContainer,
               class Compare = less<typename KeyContainer::value_type>>
      flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare = Compare())
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         Compare,
                         KeyContainer,
                         MappedContainer>;

      template<class KeyContainer, class MappedContainer, class Allocator>
      flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Allocator)
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         less<typename KeyContainer::value_type>,
                         KeyContainer,
                         MappedContainer>;
      template<class KeyContainer, class MappedContainer, class Compare, class Allocator>
      flat_multimap(sorted_equivalent_t, KeyContainer, MappedContainer, Compare, Allocator)
        -> flat_multimap<typename KeyContainer::value_type,
                         typename MappedContainer::value_type,
                         Compare,
                         KeyContainer,
                         MappedContainer>;

      template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
      flat_multimap(InputIter, InputIter, Compare = Compare())
        -> flat_multimap</*iter-key-type*/<InputIter>,
                         /*iter-mapped-type*/<InputIter>,
                         Compare>;

      template<class InputIter, class Compare = less</*iter-key-type*/<InputIter>>>
      flat_multimap(sorted_equivalent_t, InputIter, InputIter, Compare = Compare())
        -> flat_multimap</*iter-key-type*/<InputIter>,
                         /*iter-mapped-type*/<InputIter>,
                         Compare>;

      template<ranges::input_range R,
               class Compare   = less</*range-key-type*/<R>>,
               class Allocator = allocator<byte>>
      flat_multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
        -> flat_multimap<
          /*range-key-type*/<R>,
          /*range-mapped-type*/<R>,
          Compare,
          vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
          vector</*range-mapped-type*/<R>,
                 /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

      template<ranges::input_range R, class Allocator>
      flat_multimap(from_range_t, R&&, Allocator) -> flat_multimap<
        /*range-key-type*/<R>,
        /*range-mapped-type*/<R>,
        less</*range-key-type*/<R>>,
        vector</*range-key-type*/<R>, /*alloc-rebind*/<Allocator, /*range-key-type*/<R>>>,
        vector</*range-mapped-type*/<R>,
               /*alloc-rebind*/<Allocator, /*range-mapped-type*/<R>>>>;

      template<class Key, class T, class Compare = less<Key>>
      flat_multimap(initializer_list<pair<Key, T>>, Compare = Compare())
        -> flat_multimap<Key, T, Compare>;

      template<class Key, class T, class Compare = less<Key>>
      flat_multimap(sorted_equivalent_t, initializer_list<pair<Key, T>>, Compare = Compare())
        -> flat_multimap<Key, T, Compare>;

      template<class Key,
               class T,
               class Compare,
               class KeyContainer,
               class MappedContainer,
               class Allocator>
      struct uses_allocator<flat_multimap<Key, T, Compare, KeyContainer, MappedContainer>,
                            Allocator>
        : bool_constant<uses_allocator_v<KeyContainer, Allocator> &&
                        uses_allocator_v<MappedContainer, Allocator>>
      {};
    }

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 24.6.4 Header `<flat_map>` synopsis [flat.map.syn]

  - 24.6.9.2 Definition [flat.map.defn]

  - 24.6.10.2 Definition [flat.multimap.defn]

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